Step 1

Melt the butter in a large Dutch oven or heavy pot over low heat. Add the onions and stir to coat with butter. Cover the pot and simmer onions gently until tender, about 25 minutes.

Step 2

Add spinach, peas, broth, salt (1/2 teaspoon) and a few grinds of pepper, and turn the heat up to bring to a boil. Reduce to a simmer and cook gently, partially covered, for 20 minutes.

Step 3

Stir in mint, cover the pot, and simmer for 5 minutes more.

Step 4

Puree the soup using an immersion blender or in a food processor, until smooth. Correct the seasoning with salt and pepper, to taste.

Step 5

Garnish, if you like, with a spring of fresh mint in each bowl or a sprinkle of chopped fresh mint and/or a swirl of cream or creme fraiche. Serve hot.